1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive employing separate read and write repeatable runout estimated values for a head having a read element offset from a write element.
2. Description of the Prior Art
FIG. 1 shows a disk 2 having an exemplary prior art disk format comprising a plurality of concentric tracks 4, where each track 4 comprises a plurality of embedded servo sectors 6. A head is actuated radially over the disk 2 in order to write and read user data along circumferential paths defined by the tracks 4. Each embedded servo sector comprises a preamble field 8 for use in synchronizing timing recovery and gain control circuitry, and a sync mark 10 for use in discerning symbol boundaries of servo data 12. At the end of each servo sector 6 are a plurality of servo bursts 14 (A, B, C, D) which are aligned at predetermined offsets from one another and which define a centerline of the track 4. As the head reads the servo bursts 14, an indication of the head""s position relative to the track""s centerline is derived and used by a servo control system to maintain the head along the desired circumferential path during write and read operations.
It is known to employ a repeatable runout (RRO) estimated value while servo controlling the head to compensate for the repeatable disturbance in the position error signal due, for example, to the eccentricities of the disk or imperfections in writing the embedded servo sectors 6 to the disk. The RRO estimated value is fed forward into the servo loop by subtracting it from the position error signal derived from the servo bursts 14 stored in the embedded servo wedges 6. The corrected position error signal is processed by a servo compensator which generates a control current for a voice coil motor (VCM).
Magneto-resistive (MR) heads comprise a MR read element spaced apart from an inductive write element. Due to the skew angle of the MR head relative to the concentric tracks stored on the disk, the read element may not align circumferentially with the write element. At very high recording densities (tracks-per-inch (TPI)), the radial offset between the read and write elements may range from a fraction of a track to several tracks depending on the radial position of the head. When writing data to the disk, the read element is maintained over a centerline of a xe2x80x9creadxe2x80x9d track while the write element writes data to a xe2x80x9cwritexe2x80x9d track (the read and write tracks may be the same track). Due to the radial offset between the read and write elements, the data may be written along a circumferential path that is offset from the centerline of the write track. During a read operation, the read element is maintained over the circumferential path of the recorded data by introducing an offset value referred to as a xe2x80x9cmicro-jogxe2x80x9d into the servo control loop. The micro-jog corresponds to the offset of the recorded data from the track""s centerline.
Because the RRO disturbance varies with the radial location of the head, the RRO disturbance will vary between read and write operations. Thus, there is a need to compensate for the different RRO disturbances that manifest during read and write operations in disk drives employing a read element offset from a write element.
The present invention may be regarded as a disk drive comprising a disk having a plurality of tracks, each track comprising a plurality of data sectors for storing user data and a plurality of embedded servo sectors for storing servo data. The disk driver further comprises a head comprising a read element and a write element, wherein the read element is offset from the write element, the write element for writing user data to the data sectors, and the read element for reading the user data stored in the data sectors and for reading the servo data stored in the embedded servo sectors. An actuator within the disk drive actuates the head radially over the disk in response to a control signal. A position error generator responsive to the servo data read from the embedded servo sectors generates a position error signal representing a position of the read element with respect to a circumferential path along a selected one of the tracks. The disk drive stores a plurality of read repeatable runout (RRO) estimated values and a plurality of write RRO estimated values. A means is provided for combining the position error signal with the read RRO estimated values to generate a corrected position error signal during a read operation, and for combining the position error signal with the write RRO estimated values to generate the corrected position error signal during a write operation. The disk drive comprises a servo compensator, responsive to the corrected position error signal, for generating the control signal applied to the actuator.
In one embodiment, the plurality of read RRO estimated values and the plurality of write RRO estimated values are stored in the servo sectors. The plurality of servo sectors of a track comprise a first interleave and a second interleave, wherein each servo sector of the first interleave stores the write RRO estimated values corresponding to the following two servo sectors, and each servo sector of the second interleave stores the read RRO estimated values corresponding to the following two servo sectors.
The present invention may also be regarded as a disk drive comprising a disk having a plurality of tracks, each track comprising a plurality of data sectors for storing user data and a plurality of embedded servo sectors for storing servo data. The disk driver further comprises a head comprising a read element and a write element, wherein the read element is offset from the write element, the write element for writing user data to the data sectors, and the read element for reading the user data stored in the data sectors and for reading the servo data stored in the embedded servo sectors. An actuator within the disk drive actuates the head radially over the disk. The plurality of servo sectors of a track comprise a first interleave and a second interleave, wherein each servo sector of the first interleave stores write repeatable runout (RRO) estimated values corresponding to the following two servo sectors, and each servo sector of the second interleave stores read RRO estimated values corresponding to the following two servo sectors. The write RRO estimated values are used to compensate for an RRO disturbance during a write operation, and the read RRO estimated values are used to compensate for an RRO disturbance during a read operation.